Fixed Point and Weak Convergence Theorems for Α, Β -Hybrid Mappings in Banach Spaces
Total Page：16
File Type：pdf, Size：1020Kb
Hindawi Publishing Corporation Abstract and Applied Analysis Volume 2012, Article ID 789043, 12 pages doi:10.1155/2012/789043 Research Article Fixed Point and Weak Convergence Theorems for α, β-Hybrid Mappings in Banach Spaces Tian-Yuan Kuo,1 Jyh-Chung Jeng,2 Young-Ye Huang,3 and Chung-Chien Hong4 1 Center for General Education, Fooyin University, 151 Jinxue Road, Daliao District, Kaohsiung 83102, Taiwan 2 Department of Finance, Nan Jeon Institute of Technology, 178 Chaoqin Road, Yenshui District, Tainan 73746, Taiwan 3 Center for General Education, Southern Taiwan University, 1 Nantai Street, Yongkang District, Tainan 71005, Taiwan 4 Department of Industrial Management, National Pingtung University of Science and Technology, 1 Shuefu Road, Neopu, Pingtung 91201, Taiwan Correspondence should be addressed to Chung-Chien Hong, [email protected] Received 28 September 2011; Accepted 29 November 2011 Academic Editor: Muhammad Aslam Noor Copyright q 2012 Tian-Yuan Kuo et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. We introduce the class of α, β -hybrid mappings relative to a Bregman distance Df in a Banach space, and then we study the fixed point and weak convergence problem for such mappings. 1. Introduction Let C be a nonempty subset of a Hilbert space H. A mapping T : C → H is said to be 1.1 nonexpansive if Tx − Ty≤x − y, for all x, y ∈ C; 1.2 nonspreading if 2Tx − Ty2 ≤Tx − y2 Ty − x2, for all x, y ∈ C,cf.1, 2; 1.3 hybrid if 3Tx − Ty2 ≤x − y2 Tx − y2 Ty − x2, for all x, y ∈ C,cf.3. Recently, Kocourek et al. 4 introduced a new class of nonlinear mappings in a Hilbert space containing the classes of nonexpansive mappings, nonspreading mappings, and hybrid mappings and established some fixed point and ergodic theorems for mappings in this new class. For α, β ∈ R, they call a mapping T : C → H 2 Abstract and Applied Analysis 1.4α, β-hybrid if 2 2 2 2 α Tx − Ty 1 − α x − Ty ≤ β Tx − y 1 − β x − y 1.1 for all x, y ∈ C. Obviously, T is nonexpansive if and only if it is 1, 0-hybrid; T is nonspreading if and only if it is 2, 1-hybrid; T is hybrid if and only if it is 3/2, 1/2-hybrid. Motivated by the above works, we extend the concept of α, β-hybrid from Hilbert spaces to Banach spaces. For a nonempty subset C of a Banach space X,aGateauxˆ differentia- ble convex function f : X → −∞, ∞ and α, β ∈ R, a mapping T : C → X is said to be ∈ R 1.5 α, β -hybrid relative to Df if there are α, β such that − ≤ − αDf Tx,Ty 1 αDf x, Ty βDf Tx,y 1 β Df x, y 1.2 ∈ for all x, y C, where Df is the Bregman distance associated with f.Sections3 and 4 are devoted to investigation of the fixed point and weak convergence problem for such type of mappings, respectively. Our fixed point theorem extends that of 4. 2. Preliminaries ∗ In what follows, X will be a real Banach space with topological dual X ,andf : X → −∞, ∞ will be a convex function. D denotes the domain of f,thatis, D x ∈ X : fx < ∞ , 2.1 ◦ and D denotes the algebraic interior of D, that is, the subset of D consisting of all those points x ∈Dsuch that, for any y ∈ X \{x}, there is z in the open segment x, y with x, z ⊆D.The ◦ topological interior of D, denoted by IntD, is contained in D . f is said to be proper provided D ∅ ∈ ≤ that / . f is called lower semicontinuous l.s.c. at x X if f x lim infy → xf y . f is strictly convex if f αx 1 − αy <αfx 1 − αf y 2.2 for all x, y ∈ X and α ∈ 0, 1. The function f : X → −∞, ∞ is said to be Gateauxˆ differentiable at x ∈ X if there is ∗ f x ∈ X such that − f x ty fx lim y, f x 2.3 t → 0 t for all y ∈ X. The Bregman distance Df associated with a proper convex function f is the function D×D → ∞ Df : 0, defined by − ◦ − Df y, x f y fx f x, x y , 2.4 Abstract and Applied Analysis 3 ◦ − − − where f x, x y limt → 0 f x t x y f x /t. Df y, x is finite valued if and only ◦ if x ∈D, compare Proposition 1.1.2 iv of 5. When f is Gateauxˆ differentiable on D, 2.4 becomes − − − Df y, x f y fx y x, f x , 2.5 D◦ × ∞ → ∞ and then the modulus of total convexity is the function νf : 0, 0, defined by ∈D − νf x, t inf Df y, x : y , y x t . 2.6 It is known that ≥ νf x, ct cνf x, t 2.7 for all t ≥ 0andc ≥ 1, compare Proposition 1.2.2 ii of 5. By definition it follows that ≥ − Df y, x νf x, y x . 2.8 ∞ → ∞ The modulus of uniform convexity of f is the function δf : 0, 0, defined by x y δ t inf fx f y − 2f : x, y ∈D, x − y ≥ t . 2.9 f 2 The function f is called uniformly convex if δf t > 0 for all t>0. If f is uniformly convex then for any ε>0 there is δ>0 such that x y fx f y f ≤ − δ 2.10 2 2 2 for all x, y ∈Dwith x − y≥ε. ◦ Note that for y ∈Dand x ∈D , we have x y f x y − x /2 − fx fx f y − 2f f y − fx − 2 1/2 2.11 ≤ − − ◦ − ≤ f y fx f x, y x Df y, x , where the first inequality follows from the fact that the function t → fx tz − fx/t is nondecreasing on 0, ∞. Therefore ≥ νf x, t δf t, 2.12 ∈D◦ ≥ whenever x and t 0. For other properties of the Bregman distance Df , we refer readers to 5. 4 Abstract and Applied Analysis ∗ The normalized duality mapping J from X to 2X is defined by ∗ ∗ ∗ ∗ Jx x ∈ X : x, x x2 x 2 2.13 for all x ∈ X. When fxx2 in a smooth Banach space, it is known that f x2Jx for x ∈ X, compare Corollaries 1.2.7 and 1.4.5 of 6. Hence we have 2 − 2 − − Df y, x y x y x, f x 2 2 y − x − 2 y − x, Jx 2.14 2 2 y x − 2 y, Jx . Moreover, as the normalized duality mapping J in a Hilbert space H is the identity operator, we have 2 2 − − 2 Df y, x y x 2 y, x y x . 2.15 Thus, in case fxx2 in a Hilbert space, 1.5 coincides with 1.4. However, in general they are different as the following example shows. Example 2.1. Let fx|x| for x ∈ R. f is a continuous convex function with ⎧ ⎪ ≥ ≤ ⎪0, if x>0,y 0orx<0,y 0, ⎨ − ≥ ≤ Df y, x ⎪ 2y, if x 0,y 0, 2.16 ⎪ ⎩⎪ 2y, if x ≤ 0,y≥ 0. Let C 0, 1 and define T : C → C by ⎧ ⎨⎪x, if x ∈ 0, 1, Tx 2.17 ⎩⎪1 , if x 0. 2 If T were α, β-hybrid for some α, β ∈ R, then we would have αTx − T02 1 − αx − T02 ≤ βTx − 02 1 − β x − 02, ∀x ∈ 0, 1. 2.18 Abstract and Applied Analysis 5 Since 2 2 1 1 2 2 2.18 ⇐⇒ αx − 1 − αx − ≤ β|x| 1 − β |x| , ∀x ∈ 0, 1 2 2 2 1 2 ⇐⇒ x − ≤ |x| , ∀x ∈ 0, 1 2.19 2 1 ⇐⇒ ≤ x, ∀x ∈ 0, 1, 4 we see that T is not α, β-hybrid for any α, β ∈ R. But some simple computations show that T is 1, 0 -hybrid relative to Df . A function g : X → −∞, ∞ is said to be subdifferentiable at a point x ∈ X if there ∗ ∗ exists a linear functional x ∈ X such that ∗ g y − gx ≥ y − x, x , ∀y ∈ X. 2.20 We call such x∗ the subgradient of g at x. The set of all subgradients of g at x is denoted ∗ by ∂gx, and the mapping ∂g : X → 2X is called the subdifferential of g. For a l.s.c. convex function f, ∂f is bounded on bounded subsets of IntD if and only if f is bounded on bounded subsets there, compare Proposition 1.1.11 of 5. A proper convex l.s.c. function f is Gateauxˆ differentiable at x ∈ IntD if and only if it has a unique subgradient at x; in such case ∂fxf x, compare Corollary 1.2.7 of 6. The following lemma will be quoted in the sequel. Lemma 2.2 see Proposition 1.1.9 of 5. If a proper convex function f : X → −∞, ∞ is Gateauxˆ differentiable on IntD in a Banach space X, then the following statements are equivalent. a The function f is strictly convex on IntD. ∈ D b For any two distinct points x, y Int , one has Df y, x > 0. c For any two distinct points x, y ∈ IntD, one has x − y, f x − f y > 0. 2.21 Throughout this paper, FT will denote the set of all fixed points of a mapping T.